1. Field of the Invention
This invention relates to solid state relay circuits and, more particularly, to solid state relay circuits having circuitry to indicate that signals have occurred to which the solid state relay circuits are to respond.
2. History of the Prior Art
A great variety of solid state relay circuits have been developed which use a power semiconductor as the output circuit switching device. Examples of such circuits and improvements thereto are disclosed in U.S. Pat. No. 4,581,540, Circuitry Overload Protected Solid State Relay, Ciro Guajardo, issued Apr. 8, 1986. Examples of other such switching circuits are shown in co-pending patent applications Ser. No. 07/316,189, entitled Circuitry for Indicating the Presence of an Overload or Short Circuit in Solid State Relay Circuits, Ciro Guajardo, filed Feb. 27, 1989; Ser. No. 07/316,241, entitled Circuitry for Protection Against Electromotively-Induced Voltage Transients in Solid State Relay Circuits, Ciro Guajardo, filed Feb. 27, 1989; Ser. No. 07/316,406, entitled Circuitry for Protecting Against Load Voltage Transients in Solid State Relay Circuits, Ciro Guajardo, filed Feb. 27, 1989.
Although such circuits operate well as power switching apparatus, there is often a need to know that such switching is taking place. For example, it is possible that although the circuits appear to be accomplishing their purpose, some form of circuit fault has disabled the actual operation so that nothing is occurring in the switching circuitry.